[Japo]

No, it's wrong, you're mounting two different things to the same drive letter C, so the second one will overwrite the first one.

First make the folder "c:\dos" in Windows. Then launch DOSBox and enter "mount C c:\dos", then "mound D e:\ -t cdrom". Then (with the CD in the tray) enter "D:" and then "install".

If you install it to "c:\sierra\birthrt" in DOSBox, in Windows you'll have it at "c:\dos\sierra\birthrt", because "mount c c:\dos" means that you mount "c:\dos" as "c:\" inside DOSBox. The c: disk inside DOSBox is different from the one outside in Windows, it can be anything.